Two of the most common filtering techniques are the drum type and the disc type. Both techniques operate on rotating cycles in which the bottom half of the drum or disc is immersed in a trough holding fluid pulp while the top half is out of the pulp. The bottom half is held under vacuum as is most of the top half. A small section of the top half near the end of the cycle is under positive pressure to blow the cake off the filter. These filters usually have a scraper near the end of the cycle to help remove the cake. In these rotary filters, approximately 50% of the cycle for any given portion of the filter is spent rotating it immersed in the pulp drawing fluids through the filter and collecting cake against it. The rest of the cycle is spent out of the pulp and is usually divided so that 40% of the entire cycle is spent continuing to draw liquid from the cake held against the filter and the last 10% is spent blowing and scraping the cake off the filter.
The rotary nature of these filters allows very little if any variation in the percentage of time in a cycle allotted to perform each function. If the rotational speed is increased, the overall time of a cycle is reduced but the percentage of the cycle spent performing each function remains the same. The half of the cycle in which the filter is immersed is usually not a requirement to form the cake on the filter, but is more of an obligatory result of the rotary design of the filter. The cake can often be formed in less than the allotted 50%. In such cases, if the percentage of the cycle spent out of the pulp could be increased, more fluid would be drawn from the cake, making it drier. It is possible to decrease the relative time of the rotating filter spent in the pulp by immersing much less than half of the filter's area. With a disc filter that rotates about an axis parallel to the surface of the pulp, this is very inefficient because the surface area against which the cake collects is greatly reduced. With drum filters, this surface area would also be greatly reduced and the overall time to filter a given amount of pulp would be increased.
Another class of filters is reciprocating ones that are moved into and out of the pulp. Such a filter is broadly disclosed in U.S. Pat. No. 3,298,524 to Gaudfrin, issued in Class 210, Subclass 138. Gaudfrin discloses a reciprocating filter that is moved about a tank and swung through a path of 90.degree. or 180.degree.. In his preferred embodiment, Gaudfrin has a round tank divided into three chambers. His filters are moved about the tank and immersed in the first chamber full of concentrated pulp or crushed beets. Fluid is drawn through the filter and cake collects on it. The filter is then lifted out of the chamber, moved about the tank, and immersed in chamber two. This chamber contains water which is drawn through the beet cake on the filter to produce a diluted liquor. The filter is soon raised and advanced to a third chamber where air is first drawn through the filter to dry the cake and then blown through it to dislodge the cake. Gaudfrin provides a mechanism to jolt the filters over the third chamber to assist in dislodging the cake. He also has a timer to synchronize the movement of the filters.
Devices like Gaudfrin's are more concerned with producing a diluted liquor than in the efficient separation of the liquid from its cake. Consequently, their timers are only used to coordinate the movement of all the filters relative to each other so that they do not jam. Devices like Gaudfrin's do not contemplate using the timer to alter the time spent in each portion of a filter's cycle. Speeding up a timer like Gaudfrin's would only alter the length of each filter's overall cycle, not the relative time spent performing each function in the cycle. These relative times are predetermined by Gaudfrin's structure.